fix(android): latch NowPlaying drag-dismiss to prevent back-stack underflow

Operator reproduced black-screen-on-resume by drag-down dismissing
the full player. Root cause: the NestedScrollConnection accumulator
crossed the dismiss threshold, called navController.popBackStack(),
reset accumulated to 0 — but the user's finger was still down and
the pop transition was still running. The next frame's onPostScroll
re-accumulated and re-fired onDismiss(), popping the screen BENEATH
NowPlaying. When that left the back stack empty the NavHost had no
destination to draw, producing a black window until the process
was killed and the activity was cold-launched.

Add a `dismissed` latch that survives until the connection is
disposed (which only happens when NowPlayingScreen leaves the
composition, i.e. the pop completes). After the latch sets we
consume the remaining drag (return `available`) so the underlying
scrollable doesn't paint over-scroll while the pop transitions.
onPreFling also bails after dismissal so the fling can't restart
the accumulator.

android/app/src/main/java/com/fabledsword/minstrel/player/ui/NowPlayingScreen.kt

@@ -189,16 +189,29 @@ private fun rememberDragDismissConnection(
     object : NestedScrollConnection {
         private var accumulated = 0f
 
+        // One-shot latch. popBackStack is async — between the first
+        // dismissal and the screen actually leaving the composition,
+        // the user's finger is still down and more onPostScroll
+        // frames arrive. Without this guard the accumulator rebuilds
+        // and onDismiss() fires a second time, popping the screen
+        // BENEATH NowPlaying. If that leaves the back stack empty
+        // the NavHost renders nothing → black screen on resume.
+        private var dismissed = false
+
         override fun onPostScroll(
             consumed: Offset,
             available: Offset,
             source: NestedScrollSource,
         ): Offset {
+            // After dismissal, eat all remaining drag so the
+            // scrollable doesn't paint over-scroll deltas during the
+            // pop transition.
+            if (dismissed) return available
             if (source != NestedScrollSource.UserInput) return Offset.Zero
             return if (available.y > 0f) {
                 accumulated += available.y
                 if (accumulated >= thresholdPx) {
-                    accumulated = 0f
+                    dismissed = true
                     onDismiss()
                 }
                 Offset(0f, available.y)
@@ -209,6 +222,7 @@ private fun rememberDragDismissConnection(
         }
 
         override suspend fun onPreFling(available: Velocity): Velocity {
+            if (dismissed) return available
             accumulated = 0f
             return Velocity.Zero
         }